The invention relates generally to an optical transmission system using wavelength multiplexing, especially to the routing used therein.
In Wavelength Division Multiplexing WDM, several independent transceiver couples use the same fibre, and each couple uses its own wavelength, which is different from the others
FIG. 1 illustrates the principle of wavelength multiplexing. Such a system is used as an example, wherein four channels are used, wherein the wavelengths used are xcex1, xcex2, xcex3 and xcex4 respectively. The transmission and reception channels use here their own optical fibres. At each end of the optical transmission line there are four transceiver units, of which the transmitter is marked generally as Tx and the receiver as Rx. Transmitter TX1 transmits on wavelength xcex1 and receiver RX1 receives on the same wavelength, but from a fibre different from the one on which the transmitter transmits. The other couples use their own wavelengths in a corresponding manner.
The wavelengths produced by the transmitters located at the left end of fibre 8 are combined in an optical multiplexer 1 and they are then conducted to the same optical fibre 8. In a similar manner, the wavelengths produced by the transmitters located at the right end of fibre 9 are combined in an optical multiplexer 3 and they are then conducted to the same optical fibre 9.
The WDM demultiplexers 2 and 4 located at opposite ends of the fibres separate the different components of the combined signal from each other. Each one of these signals is expressed by its own receiver RX1, . . . RX4. Each optical channel may include hundreds and even thousands of logical channels.
Thus, a narrow wavelength window in a certain wavelength range is made available to the signal of each source. The International Telecommunication Union ITU-T has standardised the frequencies to be used on the band in such a way that the band begins from a frequency of 191,5 THz (1565,50 nm) and continues in steps of 100 GHz up to a frequency of 195,9 THz (1530,33 nm).
In order to achieve a maximum benefit relating to the optical transmission system, cross-connection functions are needed in the nodes of the network to route signals with different wavelengths to different paths. Components performing such a function are the multiplexerd/demultiplexer, the add/drop multiplexer, the wavelength switch and the wavelength converter. The original purpose of the multiplexer was, just as shown in FIG. 1, to increase the capacity of the existing fibre without adding any new fibres. However, they can work as an add/drop multiplexer, that is, as an access point for the optical layer and as a separation point for the optical layer as well as a part of the cross-connection arrangement.
The optical cross-connect is a small basic part of the optical coupler matrix. Its task is to route the arriving wavelength into one or more physical output ports. The cross-connect may be implemented with a directional coupler. By combining a great number of such 2xc3x972 cross-connects one another by waveguides, it is possible to build a nxc3x97n coupler matrix. The cross-connect may also be implemented by using a Semiconductor Optical Amplifier SOA.
The wavelength converter converts the wavelength of the arriving signal into another wavelength, and the converter is needed when such systems are fitted together, wherein different wavelengths are used. Available converters are opto-electronic ones, wherein the incoming optical signal is converted into electrical form, it is regenerated and transmitted further by a transmitter using a fixed wavelength.
FIG. 2 shows the principal structure of a certain multiplexer/demultiplexer. It contains port-specific lenses and an interference filter marked F. The input fibre is connected to port named xe2x80x9ccommon portxe2x80x9d, and four wavelength channels )xcex1, . . . xcex4 use that fibre in this example. Wavelength xcex1 (channel 1) goes through lens 21, through interference filter 27 and through lens 22 directly to the output port. The remaining wavelength channels are reflected back from filter 27 and they go forward to filter 28. This filter transmits wavelength xcex2 which is conducted to lens 23. Thus the channel 2 is separated. In this way, the incoming light signal is reflected back and forth within the multiplexer, and at every reflection point one exactly defined wavelength is admitted through the filter.
In the nodes of the optical network various cross-connections must be performed associated with routing, just like in electric transmission networks. In cross-connection, all wavelengths of the input fibre can be connected to the same output fibre, or certain wavelengths of the input fibre can be connected to certain output fibres. The perfect cross-connection device is an exchange, wherein all connecting functions are optical. At the present time, optical cross-connection devices are still scanty available, so in present systems the signal arriving from the optical link is in fact converted into electrical form. The necessary connections are made in electrical form, and only when the information is transferred to the output link it is converted into optical form.
FIG. 3 shows some nodes of an optical transmission network. There the node 31 multiplexes four channels to an optical link by using a coupler 35. It is important to notice that the coupler is not a wavelength-selective component, but each entry of it can be input any wavelength, and the wavelength is then summed with other wavelengths to the same fibre.
The sum signal arrives at an optical add/drop multiplexer 33 located at the middle of the link. It is a device with which one or more of the wavelengths transmitting in the fibre can be separated aside and with which one or more wavelengths can be brought to the fibre. In the arrangement shown in the figure, four optical channels are transmitted to the fibre in transmission node 31: channel 1, wherein the information is transmitted on wavelength xcex1, channel 2, wherein wavelength xcex2 is used, channel 3, wherein wavelength xcex3 is used, and channel 4, wherein the information is transmitted on wavelength xcex4. Using the add/drop multiplexer 33, channel 1, that is, wavelength xcex1 is separated and the signal content to be transported on this wavelength is conducted further in fibre 33 towards node 34. The other wavelengths continue further to node 32, wherein the wavelength components are demultiplexed and they are conducted for further processing. Connections to other nodes also start out from the node (not shown).
In the arrangement outlined in the figure, all components except coupler 35 of node 31 are wavelength-sensitive. This means that e.g. in the case of the multiplexer of node 32 an exact predetermined wavelength is obtained from each output port. FIG. 2 and the related text clarify why this is so. Likewise, in the case of the add/drop multiplexer, an exact predetermined wavelength is obtained from its output port, wavelength xcex1 in FIG. 3. When it is known that the information content of channel 1 is transmitted on wavelength xcex1, it is thus known that channel 1 is conducted to node 34. If the mode of transmission is e.g. SDH, the channel may contain hundreds or thousands of sub-channels.
The use of entirely optical network components causes a difficulty. Due to wavelength-sensitivity the routing of channels is difficult in the network. If, for example, it would be desirable in the add/drop multiplexer shown in FIG. 3 to separate channel 3 to conduct it further to node 34, then the necessary cross-connections must be made in the multiplexer. If the multiplexer is entirely optical, quite extensive changes must be made, the entire component must be exchanged in practice. Correspondingly, when wishing to replace the channel obtainable from a certain output port of demultiplexer 36, in other words, when wishing to route the channel to another output port, changes must be made in the demultiplexer, in the worst case it must be exchanged for another.
The objective of the present invention is to achieve such a channel routing method for use in a wavelength-sensitive optical network, wherein no changes at all need to be made in existing optical network components.
The established aim is achieved with the attributes described in the independent claims.
The invention is based on the realisation that instead of using optical or electrical cross-connectors in order to re-route a certain signal content to another destination, the transmission wavelength of the signal is changed, and the changed wavelength is used to transmit the signal content. Hereby the new signal content is transmitted in the transmission network on the same wavelength used for transmitting the old signal content. Since the components of the transmission network are wavelength-selective, no changes need be made in the components, but in accordance with the earlier routing plan they transmit the old wavelength to the same destination as before, except the signal content to be transported in the wavelength has in fact been exchanged for another. That element at the transmission end which combines different channels onto the same fibre may not be wavelength-selective.